Composition of, and method for forming, a semiconductor structure with multiple insulator coatings

ABSTRACT

A semiconductor structure includes a nanocrystalline core comprising a first semiconductor material, and at least one nanocrystalline shell comprising a second, different, semiconductor material that at least partially surrounds the nanocrystalline core. The nanocrystalline core and the nanocrystalline shell(s) form a quantum dot. An insulator layer encapsulates the quantum dot to create a coated quantum dot, and at least one additional insulator layer encapsulates the coated quantum dot.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part, and claims the benefit of,U.S. non-provisional patent application Ser. No. 15/154,766, filed May13, 2016, which claims priority to U.S. provisional application No.62/161,178, filed May 13, 2015, the entire contents of which are herebyincorporated by reference herein.

BACKGROUND

What is needed is a chemical composition of quantum dots, and method offorming the same, that protects the quantum dots, for example, fromwater vapor and oxygen, in order to extend the lifetime of quantumdot-based lighting and display devices, as well as other devices thatinclude quantum dots.

SUMMARY

A semiconductor structure is fabricated by first forming ananocrystalline core from a first semiconductor material, then forming ananocrystalline shell from a second, different, semiconductor materialthat at least partially surrounds the nanocrystalline core. Thenanocrystalline core and the nanocrystalline shell form a quantum dot.Multiple insulating layers are then formed, encapsulating the quantumdot.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an image from a Transmission Electron Microscope (TEM) of aquantum dot coated with an insulator layer prior to further coating thequantum dot with another insulator layer according to an embodiment ofthe invention.

FIGS. 2A, 2B, 2C, 2D, 2E and 2F are a flow diagrams in accordance withvarious embodiments of the invention.

FIG. 3 provides TEM images 300 of quantum dot particles coated withmetal oxide layers of varying thickness.

FIG. 4 provides an example of quantum dots with an insulator layeraccording to an embodiment of the invention.

FIG. 5 is an illustration of a semiconductor structure that has ananocrystalline core and nanocrystalline shell pairing with onecompositional transition layer, in accordance with an embodiment of theinvention.

FIG. 6 illustrates a nano-particle in accordance with an embodiment ofthe invention.

FIG. 7 illustrates a coated quantum dot fabricated according to anembodiment of the invention.

DETAILED DESCRIPTION

A semiconductor structure is fabricated by first forming ananocrystalline core from a first semiconductor material, then forming ananocrystalline shell from a, different, semiconductor material that atleast partially surrounds the nanocrystalline core. Additionalnanocrystalline shells may also be formed that surround the core/shellpairing. The nanocrystalline core and the nanocrystalline shell(s) forma quantum dot. Multiple insulating layers are then formed, encapsulatingthe quantum dot in an insulating structure. In an embodiment, theinsulating layers comprise inorganic material. By applying multipleinsulating layers to individually encapsulated quantum dots, defectswhich might span the thickness of the individual insulating layers areinterrupted and do not channel continuously through the insulatingstructure comprising all the multiple insulating layers. The multipleinsulating layer embodiment creates a tortuous path resulting in verylong effective diffusion pathways for environmental degradants, therebyincreasing the lifetime of the quantum dot, as well as any device thatincludes the quantum dot.

Semiconductor structures having a thick insulating structure comprisedof a plurality of insulating layers (a “thick insulator coating”) andmethods of fabricating semiconductor structures having a thick insulatorcoating are described. In an example, a method of coating asemiconductor structure with multiple layers of silica is described,including optional acid or base treatments between layers.

It has been found that quantum dots treated with multiple rounds ofinsulator coating, with base treatment in between the rounds in oneembodiment, exhibit improved thermal stability, high temperaturereliability, and/or high humidity reliability, all of which aretypically desired for good performance in a light-emitting-diode (LED)package.

Quantum dots are materials which are beneficial in many applications,but which often cannot withstand thousands of hours of operation underthe environmental and operating conditions of many products, forexample, light emitting diode (LED) or solar devices. According toembodiments of the invention, quantum dots are made robust for certainapplications by individually coating the surfaces of the quantum dotswith layers of metal oxide (for example silica, titania, alumina, etc.).An example of quantum dots with a single insulator layer is describedbelow with reference to FIG. 4. However, the single layer may not besufficient to protect the quantum dots in all operating or environmentalconditions, due to the imperfect or porous coverage of the metal oxide.Adding additional layers of metal oxide or other insulating materialmakes the quantum dots more robust by further protecting the surfacesand filling in any imperfections or pores.

Additionally, in order to ensure that there is no self-quenching ofphotoluminescence or other interactions between or among quantum dots,in one embodiment, the first metal oxide insulator layer serves as anadjustable spacer that allows the quantum dots to remain fully dispersedand spaced apart prior to adding a second metal oxide coating. By addingthe first metal oxide layer, for example, using a sol-gel process,whether by a reverse micelle, or similar, sol-gel process, theindividual quantum dots 100 are coated with enough material 110 toensure adequate monodispersity, as seen in the Transmission ElectronMicroscope (TEM) image in FIG. 1, and avoid self-quenching.

Finally, metal oxide coating of quantum dots prior to adding furtherinsulator coatings renders the quantum dots more thermally stable sothey can sustain a higher processing temperature than uncoated quantumdots.

In one embodiment of the invention 200A, with reference to the flowdiagram in FIG. 2A, and the structure 700 illustrated in FIG. 7, thefabrication of quantum dots, comprising core 705 and shell 710, withmultiple insulator coatings 715 for use in, as an example, on-chip lightemitting diode applications, is described. A number of quantum dots aresubjected to multiple rounds of silica coating or other insulatormaterial, for example, by sol-gel process, combined with base treatmentafter each round of coating. Quantum dots are coated with a firstinsulating layer 715 at 205A. This first insulating layer may be addedvia a reverse micelle method, a direct micelle method, or some othersol-gel process. These coated quantum dots are then treated at 210A withbase (caustic), followed at 215A by removing excess or un-reacted baseto ensure that a correct amount of base, which acts as a catalyst, isused for adding another insulating layer. Finally, one or moreinsulating layers 715 are added at 220A, with a base treatment aftereach round of coating. There are several methods from which to select toadd each subsequent layer, according to embodiments of the invention,including (1) reverse micelle, e.g., using Igepal to form the reversemicelle, (2) direct micelle, e.g., using AOT, (3) the Stober sol-gelprocess, (4) and an acid-catalyzed sol-gel process by acidifying anaqueous solution of metal silicate, or any other gel forming methodsknown to those skilled in the art.

For example, a first layer of silica may be formed around individualquantum dots using a sol-gel process, such as a reverse micelle sol-gelreaction. After purification, the silica coated quantum dots are treatedwith base or a mixture of different bases either by adding base(s)directly to a concentrated quantum dot stock solution in solvent or to adiluted solution of quantum dots. After base treatment, excess orun-reacted base is removed by one or more rounds of wash with anappropriate solvent, such as methanol and ethanol. More silica layerscan then be grown on the base-treated silica coated quantum dots. Thesubsequent silica coating additions can be accomplished in several ways:(1) by a reverse micelle sol-gel process as described in U.S. patentapplication Ser. No. 13/485,756; (2) by a direct micelle sol-gel processas described in US patent application Ser. No. 13/972,700, or (3) by aStober sol-gel process.

In another embodiment 200B, with reference to the flow diagram in FIG.2B, and the structure 700 illustrated in FIG. 7, involving thefabrication of quantum dots with multiple insulating coatings foron-chip light emitting diode applications, a plurality of quantum dotsis subjected to multiple rounds of sol-gel processing combined with acidtreatment, as opposed to base treatment, after each round of processing.

In this embodiment, a first layer of silica is formed around the quantumdots at 205B, using a sol-gel process, for example, using a reversemicelle sol-gel reaction. After purification, the silica shelled quantumdots are then treated with an acid or a mixture of different acidseither by adding acid(s) directly to a concentrated quantum dot stocksolution in solvent or to a diluted solution of quantum dots, at 210B.After acid treatment, excess or un-reacted acid is removed at 215B withone or more rounds of wash with an appropriate solvent, such as methanoland ethanol, ensuring that a correct amount of catalyst base is used forthe application of additional layers. Next, at 220B, one or more silicalayers can be grown on the acid treated silica layered quantum dots. Thesilica layer addition can be accomplished in several ways: (1) by areverse micelle sol-gel process; (2) by a direct micelle sol-gel; (3) bya Stober sol-gel process, or (4) by a sol-gel process that acidifies theaqueous solution of metal silicate.

In yet another embodiment 200C, with reference to the flow diagram inFIG. 2C, and the structure 700 illustrated in FIG. 7, involving thefabrication of quantum dots with multiple insulator coatings for on-chiplight emitting diode applications, a plurality of quantum dots issubjected to multiple rounds of insulator coating by sol-gel processescombined with no chemical treatment after each round of coating.

In this embodiment, a first layer of silica is formed around the quantumdots at 205C, for example, using a sol-gel process, such as a reversemicelle sol-gel reaction. The silica coated quantum dots are thenpurified at 207C. After purification, the silica coated quantum dots aresubjected at 315C to another round or more of silica coating by any ofthe following ways: (1) by a reverse micelle sol-gel; (2) by a directmicelle sol-gel process; (3) by a Stober sol-gel process, or (4) by asol-gel process that acidifies the aqueous solution of metal silicate.

In yet another embodiment 200D, with references to the flow diagram inFIG. 2D, and the structure 700 illustrated in FIG. 7, involving thefabrication of quantum dots with multiple insulating coatings foron-chip light emitting diode applications, a plurality of quantum dotsis subjected to multiple rounds of silica coating by sol-gel processes,where certain layers are treated by caustics after the addition of thelayer is complete, and others are not.

In this embodiment, a layer of silica is formed at 205D around each ofthe quantum dots using a sol-gel process, such as a reverse micellesol-gel reaction. At this point the layer can then be treated with base,or acid, or may receive no treatment, at 310D. If the first insulatinglayer is treated with base or acid, the process continues at 315D withremoval of any excess base or acid. Otherwise, or next, as the case maybe, at 220D, the silica coated quantum dots are subjected to another oneor more rounds of silica coating by any of the following ways: (1) by areverse micelle sol-gel process; (2) by a direct micelle sol-gelprocess; (3) by a Stober sol-gel process, or (4) by a sol-gel processthat acidifies the aqueous solution of metal silicate. The additionallayers of silica coating can be treated again with base or acid or mayreceive no treatment and additional layers of silica coating can beadded using any of the above methods.

Other possible ways to add the insulating layer are as follows (usingsilica as an example but not limited to silica): 1) during the silicacoating process, a silica gel forming precursor can be injected at once;2) during the silica coating process, a silica gel forming precursor canbe injected at multiple times; or 3) during the silica coating process,a silica gel forming precursor can be injected using a syringe pump atdesired rate. FIG. 2 is an image from a TEM of coated quantum dotsresulting from using a syringe pump.

In another embodiment, multiple insulating layers are applied, in whichthe multiple layers (“multilayers”) consist of alternating organic andinorganic layers that encapsulate the metal-oxide coated quantum dots.Examples of multilayer encapsulation include alternating layers ofinorganic materials such as Al₂O₃, MgO, and SiO_(x), and SiN_(x) as wellas transition metals including copper, cobalt, and iron deposited byALD. Parylene is an exemplary organic layer in multilayer structures andmay serve as the final layer due to its low modulus and hydrophobicnature. Also, a final layer of Parylene keeps water from condensing onthe Al₂O₃ layer, which is known to corrode Al₂O₃. Being a relativelyflexible polymer, Parylene may also help with stress relaxation duringthe addition of the multiple inorganic layers. Parylene can beintroduced by vapor-phase deposition between or after the addition ofthe inorganic layers. The Parylene deposition can be done at roomtemperature, eliminating any risk of thermal damage to the samples. FIG.3 provides TEM images 300 of quantum dot particles coated with metaloxide layers of varying thickness. Parylene is the trade name for avariety of chemical vapor deposited poly(p-xylylene) polymers.

In one embodiment, water-absorbing polymers can be used as organiclayers to protect quantum dots from humid environments. Water absorbingpolymers include but are not limited to poly(vinyl alcohol) PVA,poly(ethylene oxide) PEO, polyacrylamide, polyacrylate, poly(acrylicacid) and partially neutralised, lightly cross-linked poly(acrylicacid). There are a number of ways to attach these water-absorbingpolymers to silica coated quantum dots. For example, a hydroxyl group(OH group) on the outer surface of a silica shell insulator layersurface can be first reacted with cyanuric chloride, a multifunctionalcrosslinker. Then PVA can be attached to the silica shell outer surfaceby reacting with cyanuric chloride. In another example, the silica shellis first modified with aminosilanes to provide amino functional groupson the outer surface. Using a classical carbodiimide coupling chemistry,poly(acrylic acid) can be attached to the silica shell outer surface.

Multiple water absorbing polymer layers can be built using astraightforward, elegant, layer-by-layer (LBL) method. According to oneembodiment, the LBL assembly includes immersing a charged substrate, forinstance, quantum dots negatively charged with poly(acrylic acid), in asolution of positively charged polyelectrolyte such aspoly(diallyldimethylammonium) (PDDA). After rinsing with water, PDDAforms a positively charged monolayer on the surface of the substrate.Immersion in a solution of negatively charged polyelectrolyte forms anew layer, thereby switching the surface charge. This cycle can berepeated as many times as desired to assemble layers of water absorbingpolymers on the quantum dots.

The above embodiments describe, with reference to FIGS. 2A-2D and thestructure 700 illustrated in FIG. 7, the fabrication of quantum dots,comprising core 705 and shell 710, with multiple insulator coatings 715for use in, as an example, on-chip light emitting diode applications. Anumber of quantum dots are subjected to multiple rounds of silicacoating or other insulator material, for example, by sol-gel process,optionally combined with acid or base treatment after each round ofcoating. Quantum dots are coated with a first insulating layer 715 at205A/205B/205C and 205D. The above embodiments describe this firstinsulating layer may be added via a reverse micelle method, a directmicelle method, or some other sol-gel process. According to anotherembodiment 200E, the method combines quantum dots, surfactants, anaminosilane, and a silica precursor in order to grow the first silicashell insulating layer. The chemicals are similar as described in theabove embodiments, but micelles are not being formed, so the embodimentdoes not involve a reverse micelle process.

In embodiment 200E, with reference to FIG. 2E, quantum dots aredispersed in cyclohexane (Cy) liquid at 205E to create a solution. Asurfactant, such as polyoxyethylene (5) nonylphenylether, is added tothe solution at 210E. Then an aminosilane is added to the quantum dotsolution at 215E.

After few minutes of mixing, an alcohol (MeOH, EtOH, IPA etc) is addedat 220E. The role of the surfactant used here is to help dispersequantum dots having non-polar hydrophobic ligands on the surface in apolar solvent like alcohol. At this point, the quantum dot solution isoptically clear. Subsequently, ammonium hydroxide and tetraorthosilicate(TEOS) are added to the quantum dot solution at 225E to start thereaction to coat the quantum dots with silica shell.

Compared to the reverse micelle sol-gel process used in the embodimentsdescribed with reference to FIGS. 2A-2D, this embodiment is moreeffective in growing large silica particles with diameters of hundredsof nanometers to a few microns. Unlike the micelle method, there is nosize limiting micelle formed in this embodiment. Therefore the silicaparticles can grow much larger.

In yet another embodiment, 200F, with reference to FIG. 2F, quantum dotswith non-polar organic ligands on the surface are exchanged with polarligands such as polyallylamine and polyethylenimine in order to grow thefirst silica shell insulating layer. These amine polymers have multiplebinding groups to allow them to hold onto the quantum dots much tighterthan the mono dentate ligands widely used in the quantum dot synthesis.Also, the amine polymers are soluble in both alcohol and water, allowingthe silica shell growth around the quantum dots by the Stober sol-gelprocess to grow large particles effectively.

In embodiment 200F, quantum dots with non-polar organic ligands on thesurface are mixed with aminosilanes with stirring at 205F.Polyethylenimine dissolved in alcohol is then added to the quantum dotsand stirred overnight at room or elevated temperature at 210F. Inanother embodiment, polyallylamine is used instead of polyethylenimine.In another embodiment, no aminosilane is added to the quantum dots at205F. Instead, the quantum dots are mixed directly with polyallyamine orpolyethylemine at 210F. After ligand exchange, excess solvents can beremoved using a rotary evaporator at 215F. To the remaining quantumdots, water or alcohol is added to disperse the quantum dots at 220F.Next, the quantum dots are purified using an ultrafiltration filter toremove excess polymer at 225F. Finally, silica shell insulating layerscan be grown around the polyamine coated quantum dots at 230F usingStober sol-gel method, by simply mixing quantum dots withtetraorthosilicate (TEOS) or any suitable precursor and ammoniumhydroxide in alcohol/water mixture.

Example of Quantum Dots with an Insulator Layer

As explained above, embodiments of the invention involve formingmultiple insulator coatings on quantum dots, including optional base oracid treatments in between coatings. The following is an example ofquantum dots that may be treated and/or coated with insulator layersaccording to the above-described methods. Although the followingexamples may occasionally refer to a first or single insulator coating,the description may apply to any of the multiple insulator coatings.Additionally, the above-described methods may apply to any type ofquantum dots, and are not limited to the below-described coated quantumdots. In a general embodiment, a semiconductor structure includes ananocrystalline core composed of a first semiconductor material. Thesemiconductor structure also includes a nanocrystalline shell composedof a second, different, semiconductor material at least partiallysurrounding the nanocrystalline core. Additional nanocrystalline shellsmay also be formed that surround the core/shell pairing. An insulatorlayer encapsulates, e.g., coats, the nanocrystalline shell andnanocrystalline core. Thus, coated semiconductor structures includecoated structures such as the quantum dots described above. For example,in an embodiment, the nanocrystalline core is anisotropic, e.g., havingan aspect ratio between, but not including, 1.0 and 2.0. In anotherexample, in an embodiment, the nanocrystalline core is anisotropic andis asymmetrically oriented within the nanocrystalline shell. In anembodiment, the nanocrystalline core and the nanocrystalline shell forma quantum dot. In another embodiment, one or more additionalsemiconductor layers may be surround the quantum dot. An insulator layermay be formed so that it encapsulates, e.g., coats, the finalsemiconductor layer. After forming the first insulator layer, the coatedquantum dot may be coated with subsequent insulator layers. In betweenthe formation of each insulator layer, the coated quantum dot mayoptionally be treated with an acid or base as described above.

With reference to the above described coated nanocrystalline core andnanocrystalline shell pairings, in an embodiment, the insulator layer isbonded directly to the nanocrystalline shell. In one such embodiment,the insulator layer passivates an outermost surface of thenanocrystalline shell. In another embodiment, the insulator layerprovides a barrier for the nanocrystalline shell and nanocrystallinecore impermeable to an environment outside of the insulator layer.

In any case, the insulator layer may encapsulate only a singlenanocrystalline shell/nanocrystalline core pairing. In an embodiment,the semiconductor structure further includes a nanocrystalline outershell at least partially surrounding the nanocrystalline shell, betweenthe nanocrystalline shell and the insulator layer. The nanocrystallineouter shell is composed of a third semiconductor material different fromthe semiconductor material of the shell and, possibly, different fromthe semiconductor material of the core.

With reference again to the above described coated nanocrystalline coreand nanocrystalline shell pairings, in an embodiment, the insulatorlayer is composed of a layer of material such as, but not limited to,silica (SiOx), titanium oxide (TiOx), zirconium oxide (ZrOx), alumina(AlOx), or hafnia (HfOx). In one such embodiment, the layer is silicahaving a thickness approximately in the range of 3-500 nanometers. In anembodiment, the insulator layer is an amorphous layer.

With reference again to the above described coated nanocrystalline coreand nanocrystalline shell pairings, in an embodiment, an outer surfaceof the insulator layer is ligand-free. However, in an alternativeembodiment, an outer surface of the insulator layer isligand-functionalized. In one such embodiment, the outer surface of theinsulator layer is ligand-functionalized with a ligand such as, but notlimited to, a silane having one or more hydrolyzable groups or afunctional or non-functional bipodal silane. In another such embodiment,the outer surface of the insulator layer is ligand functionalized with aligand such as, but not limited to, mono-, di-, or tri-alkoxysilaneswith three, two or one inert or organofunctional substituents of thegeneral formula (R1O)3SiR2; (R1O)2SiR2R3; (R1O) SiR2R3R4, where R1 ismethyl, ethyl, propyl, isopropyl, or butyl, R2, R3 and R4 are identicalor different and are H substituents, alkyls, alkenes, alkynes, aryls,halogeno-derivates, alcohols, (mono, di, tri, poly) ethyleneglycols,(secondary, tertiary, quaternary) amines, diamines, polyamines, azides,isocyanates, acrylates, metacrylates, epoxies, ethers, aldehydes,carboxylates, esters, anhydrides, phosphates, phosphines, mercaptos,thiols, sulfonates, and are linear or cyclic, a silane with the generalstructure (R1O)3Si—(CH2)n-R—(CH2)n-Si(RO)3 where R and R1 is H or anorganic substituent selected from the group consisting of alkyls,alkenes, alkynes, aryls, halogeno-derivates, alcohols, (mono, di, tri,poly) ethyleneglycols, (secondary, tertiary, quaternary) amines,diamines, polyamines, azides, isocyanates, acrylates, metacrylates,epoxies, ethers, aldehydes, carboxylates, esters, anhydrides,phosphates, phosphines, mercaptos, thiols, sulfonates, and are linear orcyclic, a chlorosilane, or an azasilane.

In another such embodiment, the outer surface of the insulator layer isligand-functionalized with a ligand such as, but not limited to, organicor inorganic compounds with functionality for bonding to a silicasurface by chemical or non-chemical interactions such as but not limitedto covalent, ionic, H-bonding, or Van der Waals forces. In yet anothersuch embodiment, the outer surface of the insulator layer isligand-functionalized with a ligand such as, but not limited to, themethoxy and ethoxy silanes (MeO)3SiAllyl, (MeO)3SiVinyl,(MeO)2SiMeVinyl, (EtO)3SiVinyl, EtOSi(Vinyl)3, mono-methoxy silanes,chloro-silanes, or 1,2-bis-(triethoxysilyl)ethane.

In any case, in an embodiment, the outer surface of the insulator layeris ligand-functionalized to impart solubility, dispersability, heatstability, photo-stability, or a combination thereof, to thesemiconductor structure. For example, in one embodiment, the outersurface of the insulator layer includes OH groups suitable for reactionwith an intermediate linker to link small molecules, oligomers, polymersor macromolecules to the outer surface of the insulator layer, theintermediate linker one such as, but not limited to, an epoxide, acarbonyldiimidazole, a cyanuric chloride, or an isocyanate.

With reference again to the above described coated nanocrystalline coreand nanocrystalline shell pairings, in an embodiment, thenanocrystalline core has a diameter approximately in the range of 2-6nanometers. The nanocrystalline shell has a long axis and a short axis,the long axis having a length approximately in the range of 6-40nanometers, and the short axis having a length approximately in therange of 1-10 nanometers greater than the diameter of thenanocrystalline core. The insulator layer has a thickness approximatelyin the range of 1-50 nanometers along an axis co-axial with the longaxis and has a thickness approximately in the range of 3-50 nanometersalong an axis co-axial with the short axis. In other embodiments, thethickness of the insulator layer may be greater than 50 nanometers, forexample, up to 500 nanometers.

A lighting apparatus may include a light emitting diode and a pluralityof semiconductor structures that, for example, act to down convert lightabsorbed from the light emitting diode. For example, in one embodiment,each semiconductor structure includes a quantum dot having ananocrystalline core composed of a first semiconductor material and ananocrystalline shell composed of a second, different, semiconductormaterial at least partially surrounding the nanocrystalline core. Eachquantum dot has a photoluminescence quantum yield (PLQY) of at least90%. Each quantum dot may optionally have additional semiconductorlayers.

As described briefly above, an insulator layer may be formed toencapsulate a nanocrystalline shell and anisotropic nanocrystallinecore. For example, in an embodiment, a layer of silica is formed using areverse micelle sol-gel reaction. In one such embodiment, using thereverse micelle sol-gel reaction includes dissolving the nanocrystallineshell/nanocrystalline core pairing in a first non-polar solvent to forma first solution. Subsequently, the first solution is added along with aspecies such as, but not limited to, 3-aminopropyltrimethoxysilane(APTMS), 3-mercapto-trimethoxysilane, or a silane comprising aphosphonic acid or carboxylic acid functional group, to a secondsolution having a surfactant dissolved in a second non-polar solvent.Subsequently, ammonium hydroxide and tetraorthosilicate (TEOS) are addedto the second solution.

Thus, semiconductor nanocrystals coated with silica according to thepresent invention may be made by a sol-gel reaction such as a reversemicelle method. As an example, FIG. 4 illustrates operations in areverse micelle approach to coating a semiconductor structure, inaccordance with an embodiment of the present invention. Referring topart A of FIG. 4, a quantum dot heterostructure (QDH) 502 (e.g., ananocrystalline core/shell pairing) has attached thereto one or more oftrioctylphosphine oxide (TOPO) ligands 404, trioctylphosphine (TOP)ligands 406, and Oleic Acid 405. Referring to part B, the plurality ofTOPO ligands 404, TOP ligands 406, and Oleic Acid 405, are exchangedwith a plurality of Si(OCH3)3(CH2)3NH2 ligands 408. The structure ofpart B is then reacted with TEOS (Si(OEt)4) and ammonium hydroxide(NH4OH) to form a silica coating 410 surrounding the QDH 402, asdepicted in part C of FIG. 4.

With reference again to the above-described method of forming coatednanocrystalline core and nanocrystalline shell pairings, i.e., coatedsemiconductor quantum dots, in an embodiment, the first and secondnon-polar solvents are cyclohexane. In an embodiment, forming thecoating layer includes forming a layer of silica and further includesusing a combination of dioctyl sodium sulfosuccinate (AOT) andtetraorthosilicate (TEOS). In another embodiment, however, forming thelayer includes forming a layer of silica and further includes using acombination of polyoxyethylene (5) nonylphenylether andtetraorthosilicate (TEOS). In another embodiment, however, forming thelayer includes forming a layer of silica and further includes usingcationic surfactants such as CTAS (cetyltrimethylammonium bromide),anionic surfactants, non-ionic surfactants, or pluronic surfactants suchas Pluronic F 127 (an ethylene oxide/propylene oxide block co-polymer)as well as mixtures of surfactants.

Upon initiation of growth of a silica coating, the final size of thatcoating may be directly related to the amount of TEOS in the reactionsolution. Silica coatings according to embodiments of the presentinvention may be conformal to the core/shell QDH or non-conformal. Asilica coating may be between about 3 nm and 500 nm nm thick. The silicacoating thickness along the c-axis may be as small as about 1 nm or aslarge as about 500 nm. The silica coating thickness along the a-axis maybe between about 3 nm and 500 nm. Once silica coating is complete, theproduct is washed with solvent to remove any remaining ligands. Thesilica-coated quantum dots can then be incorporated into a polymermatrix or undergo further surface functionalization. However, silicalayers according to embodiments of the present invention may also befunctionalized with ligands to impart solubility, dispersability, heatstability and photo-stability in the matrix.

In another aspect, quantum dot composite compositions are described. Forexample, the quantum dots (including coated quantum dots) describedabove may be embedded in a matrix material to make a composite using aplastic or other material as the matrix. In an embodiment, compositecompositions including matrix materials and silica coated core/shellquantum dots having photoluminescence quantum yields between 90 and 100%are formed. Such quantum dots may be incorporated into a matrix materialsuitable for down converting in LED applications.

In another example, and as illustrated in FIG. 5 below, a semiconductorstructure has a nanocrystalline core and nanocrystalline shell pairingwith one compositional transition layer, in accordance with anembodiment of the present invention.

Referring to FIG. 5, a semiconductor structure 500 includes ananocrystalline core 502 composed of a first semiconductor material. Ananocrystalline shell 504 composed of a second, different, semiconductormaterial at least partially surrounds the nanocrystalline core 502. Acompositional transition layer 510 is disposed between, and in contactwith, the nanocrystalline core 502 and nanocrystalline shell 504. Thecompositional transition layer 510 has a composition intermediate to thefirst and second semiconductor materials.

In an embodiment, the compositional transition layer 510 is an alloyedlayer composed of a mixture of the first and second semiconductormaterials. In another embodiment, the compositional transition layer 510is a graded layer composed of a compositional gradient of the firstsemiconductor material proximate to the nanocrystalline core 502 throughto the second semiconductor material proximate to the nanocrystallineshell 504. In either case, in a specific embodiment, the compositionaltransition layer 510 has a thickness approximately in the range of 1.5-2monolayers. Exemplary embodiments include a structure 500 where thefirst semiconductor material is cadmium selenide (CdSe), the secondsemiconductor material is cadmium sulfide (CdS), and the compositionaltransition layer 510 is composed of CdSexSy, where 0<x<1 and 0<y<1, orwhere the first semiconductor material is cadmium selenide (CdSe), thesecond semiconductor material is zinc selenide (ZnSe), and thecompositional transition layer 510 is composed of CdxZnySe, where 0<x<1and 0<y<1.

In an embodiment, the nanocrystalline shell 504 completely surrounds thenanocrystalline core 502, as depicted in FIG. 5. In an alternativeembodiment, however, the nanocrystalline shell 504 only partiallysurrounds the nanocrystalline core 502, exposing a portion of thenanocrystalline core 502. Furthermore, in either case, thenanocrystalline core 502 may be disposed in an asymmetric orientationwith respect to the nanocrystalline shell 504. In one or moreembodiments, semiconductor structures such as 500 are fabricated tofurther include a nanocrystalline outer shell 506 at least partiallysurrounding the nanocrystalline shell 504. The nanocrystalline outershell 506 may be composed of a third semiconductor material differentfrom the first and second semiconductor materials, i.e., different fromthe materials of the core 502 and shell 504. The nanocrystalline outershell 506 may completely surround the nanocrystalline shell 504 or mayonly partially surround the nanocrystalline shell 504, exposing aportion of the nanocrystalline shell 504. Lastly, an insulator layer 508encapsulates the shell 506. In one embodiment, multiple insulator layersmay be applied, as described elsewhere herein.

In another embodiment, a network of quantum dots may be formed by fusingtogether the insulator coatings of a plurality of insulator coatedquantum dots. For example, in accordance with an embodiment of thepresent invention, insulator coatings of discrete passivated quantumdots are fused together to form a substantially rigid network of quantumdots where each quantum dot is isolated from other quantum dots in thenetwork by the fused insulator coating. In one such embodiment, fusingtogether the insulator coatings of discretely passivated quantum dotsinto a fused network provides improved optical and reliabilityperformance of the resulting structure as compared with the startingdiscretely passivated quantum dots. In one such embodiment, a chemicalbase is used to improve the optical performance of silica coatedmaterials by enabling the fusing of the insulator coatings surrounding aplurality of quantum dots. In a specific embodiment, the insulatorcoatings is a silica coating and a base such as potassium hydroxide(KOH) is used to fuse together the silica coatings of a plurality ofindividually and discretely coated quantum dots. The result is asubstantially rigid silica-based network of quantum dots. The amount ofbase material is scaled with the amount of silica in the reaction. Ingeneral, the approaches described herein have important applications forimproving the optical and reliability performance of quantum dots oreven other phosphor materials having an insulator coating and which areembedded in a matrix. In one such embodiment, the quantum dots or otherphosphor materials are first individually coated with one or moreinsulator layers and then the coated materials are fused to form aninsulator network that can be embedded in a matrix. In otherembodiments, the insulator network is formed directly on the quantumdots or other phosphor materials.

In an embodiment, then, with respect to using colloidal semiconductornanocrystals, also known as quantum dots, as downshifting fluorescentmaterials for LED lighting and/or display technologies, quantum dots areindividually coated with a silica insulator layer. The presence of thesilica coating improves the performance of the quantum dots when theyare subsequently embedded in a polymer film and subjected to variousstress tests. Applications include LED lighting applications and/ordisplay configurations. The use of base (such as KOH, NaOH or othersimilar materials) provides a fused network of the silica coated quantumdots to improve the optical performance of quantum dot materials. Asdescribed below, in particular embodiments, the scaling of the amount ofKOH or other base with silica content is balanced to achieve optimalperformance of the coated/fused quantum dots.

In an embodiment, a method of fabricating a semiconductor structureinvolves forming a mixture including a plurality of discretesemiconductor quantum dots. Each of the plurality of discretesemiconductor quantum dots is discretely coated by an insulator layer.The method also involves adding a base to the mixture to fuse theinsulator layers of each of the plurality of discrete quantum dots,providing an insulator network. Each of the plurality of discretesemiconductor quantum dots is spaced apart from one another by theinsulator network. The base may be comprised of, but not limited to,LiOH, RbOH, CsOH, MgOH, Ca(OH)2, Sr(OH)2, Sa(OH)2, (Me)4NOH, (Et)4NOH,or (Bu)4NOH.

In another embodiment, a method of fabricating a semiconductor structureinvolves forming a mixture including a plurality of discretesemiconductor quantum dots. Each of the plurality of discretesemiconductor quantum dots is discretely coated by an insulatormaterial. The method also involves adding a base to the mixture to fusethe insulator coating of each of the plurality of discrete quantum dots,providing an insulator network. Each of the plurality of discretesemiconductor quantum dots is spaced apart from one another by theinsulator network. The base may be comprised of, but not limited to,LiOH, RbOH, CsOH, MgOH, (Me)4NOH, (Et)4NOH, or (Su)4NOH, and adding thebase to the mixture involves adding one mole of the base for every twomoles of the insulator material. The method also involves adding freesilica to the mixture.

In another embodiment, a method of fabricating a semiconductor structureinvolves forming a mixture including a plurality of discretesemiconductor quantum dots. Each of the plurality of discretesemiconductor quantum dots is discretely coated by an insulatormaterial. The method also involves adding a base to the mixture to fusethe insulator coating of each of the plurality of discrete quantum dots,providing an insulator network. Each of the plurality of discretesemiconductor quantum dots is spaced apart from one another by theinsulator network. The base may be comprised of, but not limited to,Ca(OH)2, Sr(OH)2 or Sa(OH)2, and adding the base to the mixture involvesadding one mole of the base for every four moles of the insulatormaterial. The method also involves adding free silica to the mixture.

In accordance with one or more embodiments herein, an alternative toaltering seed size for tuning the emission of a seeded rod emitterarchitecture is provided. More particularly, instead of changing seedsize, the seed composition is changed by alloying either the entire seed(in one embodiment) or some portion of the seed (in another embodiment)with a higher bandgap material. In either case, the general approach canbe referred to as an alloying of the seed or nanocrystalline coreportion of a heterostructure quantum dot. By alloying the seed ornanocrystalline core, the bandgap can be changed without changing thesize of the seed or core. As such, the emission of the seed or core canbe changed without changing the size of the seed or core. In one suchembodiment, the size of the seed is fixed at the optimum size of ared-emitting seed, or roughly 4 nanometers. The fixed sized means thatthe size of the rod and the subsequent synthetic operations may not needto be substantially re-optimized or altered as the emission target ofthe quantum dots is changed.

Accordingly, in one or more embodiments described herein, optimumphysical dimensions of a seeded rod are maintained as constant whiletuning the emission peak of the heterostructure quantum dot. This can beperformed without changing the dimensions of the seed (and therefore therod) for each emission color. In a particular embodiment, a quantum dotincludes an alloyed Group II-VI nanocrystalline core. The quantum dotalso includes a Group II-VI nanocrystalline shell composed of asemiconductor material composition different from the alloyed GroupII-VI nanocrystalline core. The Group II-VI nanocrystalline shell isbonded to and completely surrounds the alloyed Group II-VInanocrystalline core. In one such embodiment, the alloyed Group II-VInanocrystalline core is composed of CdSenS1-n (0<n<1), and the GroupII-VI nanocrystalline shell is composed of CdS. In a specificembodiment, the alloyed Group II-VI nanocrystalline core has a shortestdiameter of greater than approximately 2 nanometers, and the quantum dothas an exciton peak less than 555 nanometers. In a particularembodiment, the alloyed Group II-VI nanocrystalline core has a shortestdiameter of approximately 4 nanometers, and the quantum dot has anexciton peak less than 555 nanometers, as is described in greater detailbelow

Perhaps more generally, in an embodiment, a quantum dot includes asemiconductor nanocrystalline core of arbitrary composition. The quantumdot also includes any number of semiconductor nanocrystalline shell(s).The semiconductor nanocrystalline shell(s) is/are bonded to andcompletely surrounds the semiconductor nanocrystalline core. In one suchembodiment, the semiconductor nanocrystalline core is composed of afirst Group II-VI material, and the binary semiconductor nanocrystallineshell is composed of a second, different, Group II-VI material. In onesuch embodiment, the first Group II-VI material is CdSenS1−n (0<n<1),and the second Group II-VI material is CdS.

One or more embodiments described herein involve fabrication of asemiconductor hetero-structure. The semiconductor hetero-structure has anano-crystalline core composed of a group semiconductor material. Anano-crystalline shell composed of a second, different, semiconductormaterial at least partially surrounds the nano-crystalline core. Forexample, the nano-crystalline shell may be composed of a different groupI-III-VI semiconductor material or of a group II-VI semiconductormaterial.

In one such embodiment, the above described nano-crystallinecore/nano-crystalline shell pairing has a photoluminescence quantumyield (PLQY) of greater than approximately 60%. In another, or same,such embodiment, the nano-crystalline core/nano-crystalline shellpairing provides a Type I hetero-structure. One or more embodimentsdescribed herein are directed to hetero-structure systems havingdistinct group I-III-VI material cores. In an exemplary embodiment, asphere or rod-shaped core/shell quantum dot is fabricated to have asharp compositional interface between the core and shell or agraded/alloyed interface between core and shell.

FIG. 6 illustrates an axial cross-sectional view (A) of a sphericalnano-particle 600, in accordance with an embodiment of the presentinvention. Referring to FIG. 6, an alloy region 606 is included betweenthe core 602 and shell 604 of 600. As shown in part (B) of FIG. 6, inone embodiment, the nano-particle 600 demonstrates type Ihetero-structure behavior, with excitons preferentially recombining inthe core 602 of the nano-crystal 600 due to the smaller, nested bandgapof the seed. Optionally, additional layers of material may be added,including additional epitaxial layers or amorphous inorganic and organiclayers. Other suitable embodiments are described below.

In an embodiment, systems described herein include a nano-crystallinecore emitter having a direct, bulk band gap approximately in the rangeof 1-2.5 eV. Exemplary cores include a group I-III-VI semiconductormaterial based on silver gallium sulfide having a stoichiometry ofapproximately AgGaS2. In one such embodiment, the nano-crystalline corehas a peak emission approximately in the range of 475-575 nanometers.

In one or more embodiments, the nano-crystalline core andnano-crystalline shell pairings described herein have a lattice mismatchof equal to or less than approximately 10%. In some embodiments, lessthan approximately 6% mismatch is preferable, but up to approximately10% can be workable. In particular embodiments, the mismatch is lessthan approximately 4% mismatch, as seen in successful Cd-based systems.

One or more embodiments described herein is directed to ahetero-structure core/shell pairing that is cadmium-free. For example,with reference to the above described nano-crystalline core andnano-crystalline shell pairings, in an embodiment, the first (core)material is a group I-III-VI semiconductor material. In one suchembodiment, the second (shell) semiconductor material is a second groupI-III-VI material. For example, a suitable I-III-VI/I-III-VI core/shellpairing can include, but is not limited to, copper indium sulfide(CIS)/silver gallium sulfide (AgGaS2), copper indium selenide(CISe)/AgGaS2, copper gallium selenide (CuGaSe2)/copper gallium sulfide(CuGaS2), or CuGaSe2/AgGaS2. In another such embodiment, the second(shell) semiconductor material is a group II-VI material. For example, asuitable I-III-VI/II-VI core/shell pairing can include, but is notlimited to, copper indium sulfide (CIS)/zinc selenide (ZnSe), CIS/zincsulfide (ZnS), copper indium selenide (CISe)/ZnSe, CISe/ZnS, coppergallium selenide (CuGaSe2)/ZnSe, CuGaSe2/ZnS, silver gallium sulfide(AgGaS2)/ZnS, AgGaS2/ZnSe, or silver gallium selenide (AgGaSe2)/ZnS,AgGaSe2/ZnSe.

In an embodiment, the semiconductor hetero-structure further includes anano-crystalline outer shell composed of a third semiconductor materialdifferent from the core and shell semiconductor materials. The thirdsemiconductor material at least partially surrounding thenano-crystalline shell and, in one embodiment, the nano-crystallineouter shell completely surrounds the nano-crystalline shell. In aparticular embodiment, the second (shell) semiconductor material onesuch as, but not limited to, zinc selenide (ZnSe), silver galliumsulfide (AgGaS2) or copper gallium sulfide (CuGaS2), and the third(outer shell) semiconductor material is zinc sulfide (ZnS).

While the shape of the core of the quantum dot depicted in FIG. 4 is athat of a rod, it is to be appreciated that the methods described hereinare not limited by the shape of the quantum dot and could be applied tocoated quantum dots of many different shapes, including but not limitedto spheres, rods, tetrapods, teardrops, sheets, etc. It is not limitedby the composition of the quantum dot and can be applied to quantum dotsmade from a single material or multiple materials in either acore/shell/optional shell/optional shell configuration or an alloyedcomposition. The semiconductor materials may be selected from the GroupII-VI compounds, Group III-V compounds, group IV-IV compounds, groupI-III-VI compounds, or any alloy thereof. More specifically thesemiconductor materials may be chosen from ZnO, ZnS, ZnSe, ZnTe, CdO,CdS, CdSe, CdTe, HgS, HgSe, HgTe, HgO, AIN, ATP, AlAs, AlSb, GaN, GaP,GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb, PbS, PbSe,PbTe, MgO, MgS, MgSe, alloys thereof, and mixtures thereof.

What is claimed is:
 1. A method of fabricating a semiconductorstructure, comprising: forming a nanocrystalline core from a firstsemiconductor material; forming at least one nanocrystalline shell froma second, different, semiconductor material that at least partiallysurrounds the nanocrystalline core, wherein the nanocrystalline core andthe nanocrystalline shell(s) form a quantum dot; forming an insulatorlayer encapsulating the quantum dot to create a coated quantum dot; andforming at least one additional insulator layer on the coated quantumdot.
 2. The method of claim 1, further forming additional insulatorlayers encapsulating the quantum dot such that defects that may exist ineach of the insulating layers are interrupted and do not channelcontinuously through the insulating layers.
 3. The method of claim 1,wherein forming the insulator layer encapsulating the quantum dot tocreate a coated quantum dot comprises forming a first metal oxide layerencapsulating the quantum dot.
 4. The method of claim 3, wherein formingthe metal oxide layer encapsulating the quantum dot comprises selectingthe metal oxide from a group of metal oxides consisting of silica(SiOx), titanium oxide (TiOx), zirconium oxide (ZrOx), alumina (AlOx),magnesium oxide (MgOx) and hafnia (HfOx).
 5. The method of claim 2,wherein forming additional insulator layers on the coated quantum dotprovides for improved thermal stability, high temperature reliability,and/or high humidity reliability of the coated quantum dot.
 6. Themethod of claim 2, wherein forming the additional insulator layers onthe coated quantum dot comprises forming organic layers and/or inorganiclayers on the coated quantum dot.
 7. The method of claim 6, whereinforming organic layers comprises forming at least one layer ofwater-absorbing polymer.
 8. The method of claim 7, wherein thewater-absorbing polymer is selected from a group consisting of:poly(vinyl alcohol) PVA, poly(ethylene oxide) PEO, polyacrylamide,polyacrylate, poly(acrylic acid) and partially neutralised, lightlycross-linked poly(acrylic acid).
 9. The method of claim 6, whereinforming inorganic layers comprises forming a layer with a compound ormetal selected from a group of compounds and metals consisting of silica(SiOx), titanium oxide (TiOx), zirconium oxide (ZrOx), alumina (AlOx),magnesium oxide (MgOx) and hafnia (HfOx), and SiN_(x) copper, cobalt,and iron.
 10. The method of claim 1, wherein forming the insulator layerencapsulating the quantum dot to create a coated quantum dot comprisesforming a first insulator layer encapsulating the quantum dot using asol-gel process; and wherein forming the additional insulator layers onthe coated quantum dot comprises forming an additional insulating layerencapsulating the quantum dot using a sol-gel process.
 11. The method ofclaim 10, wherein forming a first insulator layer encapsulating thequantum dot using a sol-gel process comprises forming the firstinsulator layer encapsulating the quantum dot using a reverse micellesol-gel process; and wherein forming the additional insulating layerencapsulating the quantum dot using a sol-gel process comprises formingthe additional insulating layer encapsulating the quantum dot using oneof: reverse micelle sol-gel process, a direct micelle sol-gel process, aStober sol-gel process, and a sol-gel process that acidifies theencapsulated quantum dot.
 12. The method of claim 1, wherein forming theinsulator layer encapsulating the quantum dot to create a coated quantumdot comprises forming a first insulator layer encapsulating the quantumdot by: dispersing the quantum dot in solution; adding surfactant to thesolution; adding aminosilane to the solution; adding alcohol to thesolution; and adding ammonium hydroxide and tetraorthosilicate to thesolution.
 13. The method of claim 1, wherein forming the insulator layerencapsulating the quantum dot to create a coated quantum dot comprisesforming a first insulator layer encapsulating the quantum dot by: mixingnon-polar ligands on a surface of the quantum dot with aminosilanes;adding polyethylenimine dissolved in alcohol to the quantum dot;dispersing the quantum dot in water or alcohol; and adding ammoniumhydroxide and tetraorthosilicate.
 14. The method of claim 1, wherein,after forming the insulator layer encapsulating the quantum dot tocreate a coated quantum dot, and prior to forming the at least oneadditional insulator layer on the coated quantum dot, treating thecoated quantum dot with a base or an acid.
 15. The method of claim 1,wherein, after forming the at least one additional insulator layer onthe coated quantum dot, and prior to further forming additionalinsulator layers encapsulating the quantum dot, treating the coatedquantum dot with a base or an acid.
 16. The method of claim 1, whereinforming the insulator layer encapsulating the quantum dot to create acoated quantum dot, or forming the at least one additional insulatorlayer on the coated quantum dot during the silica coating process,comprises one of: injecting a silica gel forming precursor at least onceduring the formation, injecting the silica gel forming precursor aplurality of times during the formation, and injecting the silica gelforming precursor using a syringe pump at desired rate.
 17. The methodof claim 1, further comprising ligand-functionalizing an outer surfaceof the insulator layer with a ligand selected from a group consisting oforganic compounds, and inorganic compounds, with ligand-functionalityfor bonding to the outer surface by chemical or non-chemicalinteractions selected from a group consisting of: covalent, ionic,H-bonding, and Van der Waals forces.
 18. The method of claim 1, furthercomprising ligand-functionalizing an outer surface of the insulatorlayer with a ligand selected from a group consisting of: methoxy andethoxy silanes (MeO)3SiAllyl, (MeO)3SiVinyl, (MeO)2SiMeVinyl,(EtO)3SiVinyl, EtOSi(Vinyl)3, mono-methoxy silanes, chloro-silanes, and1,2-bis-(triethoxysilyl)ethane.
 19. A semiconductor structure,comprising: a nanocrystalline core comprising a first semiconductormaterial; at least one nanocrystalline shell comprising a second,different, semiconductor material that at least partially surrounds thenanocrystalline core, wherein the nanocrystalline core and thenanocrystalline shell(s) form a quantum dot; an insulator layerencapsulating the quantum dot to create a coated quantum dot; and atleast one additional insulator layer encapsulating the coated quantumdot.
 20. The apparatus of claim 19, further comprising additionalinsulator layers encapsulating the quantum dot such that defects thatmay exist in each of the insulating layers are interrupted and do notchannel continuously through the insulating layers.
 21. The apparatus ofclaim 19, wherein the insulator layer encapsulating the quantum dot tocreate a coated quantum dot comprises a first metal oxide layerencapsulating the quantum dot.
 22. The apparatus of claim 21, whereinthe metal oxide layer encapsulating the quantum dot is selected from agroup of metal oxides consisting of silica (SiOx), titanium oxide(TiOx), zirconium oxide (ZrOx), alumina (AlOx), magnesium oxide (MgOx)and hafnia (HfOx).
 23. The apparatus of claim 20, wherein the additionalinsulator layers on the coated quantum dot provide for improved thermalstability, high temperature reliability, and/or high humidityreliability of the coated quantum dot.
 24. The apparatus of claim 20,wherein the additional insulator layers on the coated quantum dotcomprise organic layers and/or inorganic layers on the coated quantumdot.
 25. The apparatus of claim 24, wherein the organic layers compriseat least one layer of water-absorbing polymer.
 26. The apparatus ofclaim 25, wherein the water-absorbing polymer is selected from a groupconsisting of: polyvinyl alcohol) PVA, poly(ethylene oxide) PEO,polyacrylamide, polyacrylate, poly(acrylic acid) and partiallyneutralised, lightly cross-linked poly(acrylic acid).
 27. The apparatusof claim 24, wherein the inorganic layers comprise a layer with acompound or metal selected from a group of compounds and metalsconsisting of silica (SiOx), titanium oxide (TiOx), zirconium oxide(ZrOx), alumina (AlOx), magnesium oxide (MgOx) and hafnia (HfOx), andSiN_(x) copper, cobalt, and iron.
 28. A lighting apparatus, comprising:a light emitting diode; and a plurality of quantum dots, each quantumdot comprising: a nanocrystalline core comprising a first semiconductormaterial; at least one nanocrystalline shell comprising a second,different, semiconductor material that at least partially surrounds thenanocrystalline core, wherein the nanocrystalline core and thenanocrystalline shell(s) form a quantum dot; an insulator layerencapsulating the quantum dot to create a coated quantum dot; and atleast one additional insulator layer encapsulating the coated quantumdot.